Microscopy101
Recipes for Consistent Selected Area Electron Diffraction Results: Part 3: Electron Diffraction Analysis Software
Scott D. Walck SURVICE Engineering Co., contractor to U.S. Army Combat Capabilities Development Command Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, MD 21001
scott.d.walck2.ctr@mail.mil
Abstract: Electron diffraction is an essential characterization tool for materials scientists. When using the transmission electron microscope (TEM) to perform diffraction experiments, setting up the microscope for both calibration standards and unknown materials in a consistent method ensures that dependable results are obtained. Care must also be exercised to protect digital cameras from intense transmitted and diffracted beams to avoid damage. In Parts 1 and 2 of this series, procedures were presented for recording high-quality, well-calibrated, digital SAED patterns. In Part 3, aspects of diffraction analysis software packages and the reliability of data are discussed.
Keywords: TEM, selected area electron diffraction, SAED, calibration
Introduction In the previous articles in this series procedures for set-
ting up the microscope and recording SAED patterns were discussed. If the procedures are followed, the resulting SAED patterns will be well-calibrated and centered with good inten- sities for analysis. Te patterns will also be well-suited for use in electron diffraction analysis soſtware packages. In this last installment, soſtware packages are discussed, and results for testing the reproducibility and reliability of SAED data acquired with these procedures are given.
Electron Diffraction Analysis Software Tere are several commercial and free soſtware packages
available for the analysis of electron diffraction patterns. Tis author regularly uses DiffTools [1,2], CSpot [3], JEMS [4], and Electron Diffraction [5] to analyze experimental spot, ring, and Kikuchi patterns. It is beyond the scope of this article to discuss all electron diffraction soſtware packages. How- ever, there are important features that they should include. Te program should be able to simulate patterns from crys- tal information that is either imported or created within the soſtware. Te simulated pattern should be able to be overlaid with the diffraction pattern. Te different types of diffraction patterns should be able to be indexed, for example, ring, spot, and Kikuchi patterns. It also helps to be able to interactively change the camera length to help match the pattern when the simulated pattern doesn’t quite fit the experimental one. For ring patterns, it is also very useful to integrate the pattern and generate a plot that can be compared to X-ray diffraction pat- terns. Tere are three critical features that must be included in the soſtware: pattern centering, correction of pattern distor- tion, and pattern calibration.
46 doi:10.1017/S1551929520001066 Pattern centering. To measure d-spacings and angles
in diffraction patterns, the center of the pattern must be accurately positioned. Te procedures outlined previously for producing a “double exposure” image provides a fiducial marker that makes pattern centering easy. DiffTools has the Locate (000) Spot command, which locates the pattern cen- ter with sub-pixel resolution by finding a geometric center of gravity for the transmitted beam. Te DiffTools – Locate SADP Centre menu command can also be used to locate the pattern center. With this tool, concentric circles can be super- imposed on the SAED pattern as an aid to check the results, as shown in Figure 1A. Te Auto method uses a Circular Hough Transform technique to accurately find the center [6]. Figure 1B shows the manual centering of the pattern in CSpot with superimposed Au rings on the pattern. Also available in CSpot is the ability to use a circular fitting method for deter- mining the center by positioning points on a diffraction ring (Figure 1C). Te colorized zoom window helps facilitate the accurate positioning of the fitting point shown in red at the maximum intensity. Correction for elliptical patterns. Not all microscopes
are created equally. Distortions are possible with the lenses that might make the patterns non-circular, that is, elliptical. Of course, this will have an adverse effect on any measurements of d-spacings or angles. Te tolerance on new microscopes is less than 1%, but this needs to be checked to see if distortions are present. Te good news is that with digital images, the distor- tion can be easily measured using a polycrystalline material that generates ring patterns, as shown throughout this article. If the rings are not circular, the pattern must be corrected, and this correction can then be applied to any diffraction pattern from an unknown material [7]. For CSpot in Figure 1C, it is seen that there are two options to find the center from the sam- ple points, fitting either a circle or an ellipse. Dave Mitchell has also created an Ellipse Fitting Analysis routine that can mea- sure the elliptical distortion, save the correction parameters, and apply it to all patterns collected subsequently [8,9]. Tis has the advantage of applying the distortion correction to any SAED pattern in DM immediately aſter it is recorded and prior to analyzing it with any soſtware package. Calibration. Obviously, the digital image must be cali-
brated in order to make measurements from it and to compare it to simulated phases. Anyone who has performed diffraction
www.microscopy-today.com • 2020 July
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